Arboviruses (ARthropod-BOrne viruses) are viruses that are spread by insects, often mosquitoes. As I’ve written about recently, mosquito biology and differences in reservoir species mean that mosquito-borne viruses can have quite different patterns of distribution and spread. As is common, the occurrence of West Nile virus infection in horses tends to ramp up in late August and into the fall, and is broadly spread across North America. In contrast, Eastern Equine Encephalitis virus (EEE), while a cause of nastier disease, tends to have a more narrow geographic range, occurring mostly along the Atlantic and Gulf Coast regions in the US. West Nile cases in particular seem to be spiking lately. Check out  Worms & Germs Map ( for the latest case maps. As always, these maps don’t show every case of disease, just those that are diagnosed and reported through various means, but they do provide an interesting snapshot of these two diseases.

Eastern Equine Encephalitis in horses, last update Aug 30, 2016

EEE copy

West Nile virus in horses. Last updated Aug 30, 2016.

WNV copy